This is "WinDump.exe udp" output, on Windows XP.

These are fragmented UDP packets, typically 10K to 30K size. They leave the XP box successfully & arrive at 75-144-20-99 successfully. When 75-144-20-99 sends a packet back, it looks like only the first fragment (with the UDP header) makes it back.

Why not the other fragments?

The same kind of fragmented UDP do arrive back -- through the exact same routers & masquarade -- on a linux box on the same network, same switch, as this Windows box. So I think the problem has to be in XP.

20:56:03.008291 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.008303 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.063024 IP 75-144-20-99-ca.sfba.comcastbusiness.net.55055 > STRICK-CORP.
mshome.net.9899: UDP, length 14248
20:56:03.215487 IP STRICK-CORP.mshome.net.9899 > 75-144-20-99-ca.sfba.comcastbus
iness.net.55055: UDP, length 21334
20:56:03.215515 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215528 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215541 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215553 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215565 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215578 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215590 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215602 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215614 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215627 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215639 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215652 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215663 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.215675 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.262692 IP 75-144-20-99-ca.sfba.comcastbusiness.net.55055 > STRICK-CORP.
mshome.net.9899: UDP, length 13950
20:56:03.426976 IP STRICK-CORP.mshome.net.9899 > 75-144-20-99-ca.sfba.comcastbus
iness.net.55055: UDP, length 21288
20:56:03.427005 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427018 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427030 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427043 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427055 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427067 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427080 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427092 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427104 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427116 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427128 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427140 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427152 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.427164 IP STRICK-CORP.mshome.net > 75-144-20-99-ca.sfba.comcastbusiness
.net: udp
20:56:03.463705 IP 75-144-20-99-ca.sfba.comcastbusiness.net.55055 > STRICK-CORP.
mshome.net.9899: UDP, length 14300